1. Field of the Invention
The present invention relates to a method of manufacturing a fibrous structure and to an apparatus therefor. The invention is particularly, though not exclusively, applicable to the manufacture of friction products such as preform discs for use in brake mechanisms, and to hollow structures such as preform cones, nozzles and the like for projectiles and aircraft.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Use of the term ‘preform’ indicates that the structures manufactured in accordance with the present invention are heated in an autoclave at an appropriate temperature for an appropriate period of time in order to produce a hardened carbonaceous structure. Such a structure is made of carbon that is reinforced with carbon fibres. Suitable carbon fibres for use in the manufacture of such a structure are well known to those in the art and are, for example, sold under the registered trade mark SIGRAFIL®. Alternatively, suitable carbonizable fibres for use in the production of the preform are also known to persons skilled in the art and may comprise oxidized polyacrylonitrile fibers or precursors of polyacrylonitrile fibers of various types such as are sold under the registered trade mark PANOX®. These fibres are converted to carbon fibres by the heat treatment and produce carbon particles which enter and fill voids in the preform to densify the structure. Both types of fibres may be produced in a continuous tow, sliver, roving or yarn form, each of which is made up of a large number of flexible filaments, for example between 30,000 and 350,000 filaments, in order that the preform can be manufactured using textile techniques. Other fibres that are impregnated with resin are also suitable for use in the production of preforms. These are then also carbonizable.
Conventional preform discs for use in brake mechanisms are manufactured in various ways. EP 0 748 781 (B.F. Goodrich) describes a method of making a preform wherein braided tapes are spirally wound and connected together by needle punching. The braid has helical fibre along its length so that none of the fibres making up the disc ever extends linearly either from the outer diameter of the disc to the inner side in a radial direction or at a tangent to the disc. The fibre always extends along a curve from the center to the circumference or vice versa.
WO 98/49382 (B. F. Goodrich) describes a method wherein a mass of loose fibre is accreted into a thick fibrous structure by repeatedly driving a multitude of felting needles into the loose fibre, the felting needles penetrating all the way through the fibrous material at the beginning of the process, and penetrating only part way through the fibrous material at the end of the process. The orientation of the loose fibre in the finished preform is, therefore, random.
EP 0 232 059 (P. G. Lawton Limited) describes a method of manufacturing a carbon fibre disc wherein the disc is built by adding segments, one by one, in a spiral, layer upon layer. Some segments have continuous filaments extending chordally and others radially. The layers of segments are connected by needling. Hence, the resulting preform has filaments that extend chordally, radially and transversely, the last being produced by the needling head.
GB 2 428 253 (P. G. Lawton (Industrial Services) Limited) describes a method of manufacturing a stack of fibrous material that is built up in the direction of the longitudinal axis of the stack by causing relative movement of a support for the stack and at least one feed of fibrous material adding fibrous material to build the stack. The stack is therefore built up in overlying layers which are connected by needling.
The last two disclosures describe the manufacture of discs and cylindrical stacks based on an ‘xyz’ principle wherein the fibres extend chordally (x direction), radially (y direction) and longitudinally (z direction) with respect to one another. Such a three dimensional configuration of fibres increases the strength of the finished product. However, it will be appreciated that in these inventions there are disrupted fibres at the inner and outer circumferences of the disc or stack which must be machined away after carbonization. Also, as the fibres in the longitudinal direction are those which are needle punched, they do not necessarily extend through the whole of the stack or disc, which is basically built up in a series of layers that are needle punched together. This can result in a stack or disc that has a propensity to fail owing to delamination.
In addition to the foregoing, none of the inventions is suited to the manufacture of hollow structures. The only way these could be manufactured would be to machine the stack of material after carbonization to reduce the cylindrical stack to the shape of the hollow structure required. This is not economically viable in most cases as most of the stack would be machined away and, therefore, wasted.